The presence of microbial cells on a metal surface, as well as their metabolic activities, can cause Microbiologically Influenced Corrosion (MIC). The forms of corrosion caused by bacteria are not unique.

Biocorrosion results in pitting, crevice corrosion, selective dealloying, stress corrosion cracking, and under-deposit corrosion.

The following mechanisms are some of the causes of biocorrosion.

Oxygen depletion or differential aeration cells

Fig 1 Nonuniform (patchy) colonization by bacteria results in differential aeration cells. This schematic shows pit initiation due to oxygen depletion under a biofilm. (Borenstein 1994)

Nonuniform (patchy) colonies of biofilm result in the formation of differential aeration cells where areas under respiring colonies are depleted of oxygen relative to surrounding noncolonized areas. Having different oxygen concentrations at two locations on a metal causes a difference in electrical potential and consequently corrosion currents. Under aerobic conditions, the areas under the respiring colonies become anodic and the surrounding areas become cathodic.

Stainless steels’ protective film

Oxygen depletion at the surface of stainless steel can destroy the protective passive film. Remember that stainless steels rely on a stable oxide film to provide corrosion resistance. Corrosion occurs when the oxide film is damaged or oxygen is kept from the metal surface by microorganisms in a biofilm.

Sulfate-reducing bacteria (SRB)

Oxygen depletion at the surface also provides a condition for anaerobic organisms like sulfate-reducing bacteria to grow. This group of bacteria are one of the most frequent causes for biocorrosion. They reduce sulfate to hydrogen sulfide which reacts with metals to produce metal sulfides as corrosion products. Aerobic bacteria near the outer surface of the biofilm consume oxygen and create a suitable habitat for the sulfate reducing bacteria at the metal surface. SRBs can grow in water trapped in stagnant areas, such as dead legs of piping. Symptoms of SRB-influenced corrosion are hydrogen sulfide (rotten egg) odor, blackening of waters, and black deposits. The black deposit is primarily iron sulfide. (Borenstein 1994 and Geesey 1994)

One way to limit SRB activity is to reduce the concentration of their essential nutrients:

• phosphorus,
• nitrogen, and
• sulfate.

Thus, purified (RO or DI) waters would have less problem with SRBs. Also, any practices which minimize biofilm thickness (flushing, sanitizing, eliminating dead-end crevices) will minimize the anaerobic areas in biofilm which SRBs need" (Geesey 1994).

Byproducts of bacterial metabolism

Another corrosion mechanism is based on the by-products of bacterial metabolism.

Acid-producing bacteria
Bacteria can produce aggressive metabolites, such as organic or inorganic acids. For example, Thiobacillus thiooxidans produces sulfuric acid and Clostridium aceticum produces acetic acid. Acids produced by bacteria accelerate corrosion by dissolving oxides (the passive film) from the metal surface and accelerating the cathodic reaction rate (Borenstein 1994).

Hydrogen-producing bacteria
Many microorganisms produce hydrogen gas as a product of carbohydrate fermentation. Hydrogen gas can diffuse into metals and cause hydrogen embrittlement.

Iron bacteria
Iron-oxidizing bacteria, such as Gallionella, Sphaerotilus, Leptothrix, and Crenothrix, are aerobic and filamentous bacteria which oxidize iron from a soluble ferrous (Fe2+) form to an insoluble ferric (Fe3+) form. The dissolved ferrous iron could be from either the incoming water supply or the metal surface. The ferric iron these bacteria produce can attract chloride ions and produce ferric chloride deposits which can attack austenitic stainless steel. For iron bacteria on austenitic stainless steel, the deposits are typically brown or red-brown mounds.